Plants working to combat pollution

Green shoots ... Bacteria in the roots of plants can help remediate polluted air or water. Photograph: Al Behrman/AP

Last year was a "pretty good" one for Phytorestore, according to CEO Thierry Jacquet. This small company is the French market leader in a promising, but still marginal sector, phytoremediation, or more simply, using plants to mitigate pollution.

"The plants are actually an alibi," says Jean-Louis Ducreux, who heads the Atelier d'Ecologie Urbaine, a consultancy specialising in these processes. "It's the bacteria attached to their roots that do the real work." The contaminants are digested or, in the case of heavy metals that cannot be broken down, trapped by the plant fibres. The type of vegetation – grass, reeds, bamboo, irises, maize, sorghum, willow, poplar – depends on their ability to break down or sequester pollutants.

The most advanced processes concern water purification. Plant-based remediation systems are becomingly increasingly common in small country towns in France. Larger towns, such as Honfleur or Caen, are using Phytorestore-designed gardens to filter water as it leaves conventional sewage plants. "At the outlet we obtain water clean enough to swim in," Jacquet says.

Soil remediation is still at the research-and-development stage. Here the aim is to fix pollutants in the contaminated soil, or to "transfer pollutants from the soil to the vegetation, using plants with a high rate of uptake which are then harvested", says Valérie Bert, a research engineer at France's Institute of the Industrial Environment and Risk Management (Ineris). This year the institute will be experimenting various techniques at Creil, north of Paris.

Research labs are still investigating comparable methods for combating air pollution.

Plant-based methods have many advantages. They generally cost less than conventional processes, particularly with respect to operating costs.

But there are drawbacks. Plants need plenty of space to filter water and time to trap pollution in the earth, a process sometimes lasting several years.

But this is not enough to explain the problems phytoremediation has had putting down roots in France. "We get the impression it is developing everywhere else but here," says Ducreux, who notes that large-scale systems have been build abroad. Half of Phytorestore's projects are outside France and most of them are larger.

"All the regulations favour the state of the art in the dominant industrial technologies," Jacquet says. But he is sure plants will be increasingly used to remove pollution. "They represent an economically competitive solution," he claims. "So I'm reasonably optimistic."